In Magnetic Resonance Imaging (MRI), contrast agents are often used to improve depiction of diseased from normal tissue. For instance, contrast agents may improve the sensitivity of MRI to identify myocardial infarction, stroke, tumors, atherosclerosis, tissue necrosis, and other diseased states. However, contrast agents may also increase blood pool signal. This may confound detection of abnormal tissue adjacent to blood. One example of this problem is in the setting of a myocardial infarction. Contrast-enhanced MRI is very useful in detecting myocardial infarction, and allows delineation of infarcted from non-infarcted regions by virtue of the fact that infarcted regions accumulate contrast media to a greater degree (see, e.g., Kim R. J. et al. (1999) Circulation 100:1992-2002). Thus, on a T1-weighted MRI image after administration of a contrast agent with high R1 relaxivity, infarcted myocardium is usually bright (high image intensity) compared with non-infarcted myocardium. Unfortunately, the administration of contrast media also leads to high levels of contrast in the blood, and often it is difficult to delineate the border of infarcted myocardium from the blood in the heart chamber cavity. This may render the infarct undetectable, if small, or at a minimum make it difficult to accurately size the infarct. Another example is in the setting of vascular disease. Differences in contrast uptake within separate components of an atherosclerotic plaque is important to identify but may be difficult to detect since the blood within the lumen is immediately adjacent. These examples concern the cardiovascular system, however, the present disclosure may improve the depiction of diseased tissue from any organ system by suppressing blood pool signal.
Traditional “black-blood” MRI techniques have been in clinical use for several years, and are valued parts of the MRI armamentarium (see, e.g., Edelman, R. R. et al. (1991) Radiology 181:655-660; Simonetti, O. P. et al. (1996) Radiology 199:49-57). Moreover, these techniques have been improved to increase efficiency (see, e.g. U.S. Pat. Nos. 6,498,946 and 7,315,756). However, these techniques were not designed to image with the use of contrast media and the pulse sequence timing is usually based on the T1 of native blood without contrast media. Thus, these techniques were not intended to provide contrast-enhanced images of tissues, and not surprisingly, work poorly after the administration of contrast.
More recently, there have been several attempts to perform contrast-enhanced MRI with the suppression of blood pool signal. These include: (1) an MRI method and apparatus to improve myocardial infarction detection with blood pool signal suppression where the pulse sequence involves the use of a “notched” inversion RF pulse (see, e.g., U.S. Pat. No. 6,526,307); (2) an MRI method to improve imaging of atherosclerotic plaque with suppression of blood signal where the pulse sequence employs a quadruple inversion-recovery (QIR) preparative pulse (see, e.g., Yarnykh, V. L. et al. (2002) Magn. Reson. Med. 48:899-905); (3) techniques for black-blood imaging of myocardial infarction (see, e.g., Rehwald, W. G. et al. (2007) Proc. Intl. Soc. Magn. Reson. Med. and Salerno, M. et al. (2007) Proc. Intl. Soc. Magn. Reson. Med; see also US 20090005673); and (4) MRI methods for black-blood delayed enhancement of myocardial imaging where the pulse sequence involves a stimulated-echo acquisition mode (STEAM) (see, e.g., Ibrahim, el S. H. et al. (2008) J. Magn. Reson. Imaging 27:229-238).
Of the above-enumerated methods, nos. 1-3 work by affecting the longitudinal magnetization of blood in regions outside of the imaged slice in a manner such that at the time when MR data is acquired, signal from blood outside of the imaged slice that flows into the specific region-of-interest is suppressed. Method 4 works in a different manner and behaves similar to spin-echo imaging with respect to blood flow. Irrespective of the specific mechanism, all of these methods are dependent on the speed of blood flow, and blood may be mistaken as part of the anatomy.
There have also been attempts to perform contrast-enhanced MRI with improved contrast between tissue and blood pool and yet in a manner that is independent of blood flow velocity. For example, Kellman et al. describe a multi-contrast MRI technique to improve contrast between myocardial infarction and blood pool by acquiring two separate images: a T2-weighted and a T1-weighted image (see, e.g., Kellman, P. et al. (2005) J. Magn. Reson. Imaging 22:605-613). Liu et al. describe a technique that is similar to that described by Kellman et al., but combines the weighting of T2 and T1 in a single image (see, e.g., Liu et al. (2008) J. Magn. Reson. Imaging 28: 1280-1286). Moreover, Foo et al. describe a dual inversion time subtraction method which utilizes two acquisitions at a long and short inversion time to improve delineation between the endocardial borders of an infarct from the ventricular blood pool (see, e.g., Liu, C. Y. et al. supra).
Although contrast between tissue and blood pool may be improved by some of the methods described in the paragraph above, the level of blood suppression may be minimal, and none of these methods are considered black-blood techniques. Specifically, these methods may result in images in which the signal of blood pool is higher than that of normal myocardium.